A gas sensor or gas concentration sensor is a subclass of chemical sensors that measures the concentration of gases in the vicinity of the sensor. A gas interacts with the sensor and provides a measure of the concentration of the gas based on a signal or property change created by the interaction. Gas sensors can include sensors based on measuring changes in electrical properties of the sensor, such as, but not limited to, metal oxide semiconductor sensors, polymer coating based sensors, carbon nanotube based sensors, and moisture adsorbing material based sensors. Other types of gas sensors can include sensors based on measuring other properties, such as, but not limited to, optical, acoustic, gravimetric, gas chromatograph, flame ionization, and calorimetric based sensors.